Lithium hydride, LiH

At 500° C. lithium combines with hydrogen, becoming coated with a superficial layer of the hydride. At bright redness the combination is complete, and is attended by incandescence. The preparation of the hydride is effected by passing a current of dry hydrogen over the heated metal below 710° C., the product being a transparent, vitreous, opalescent mass, with the formula LiH. On exposure to light it acquires a blue colour, without change in composition. Its melting-point is 680° C., its dissociation-pressure at this temperature being about 27 mm. The density of the hydride is 0.816, and its molecular volume 9.77.

The alkali-metal and alkaline-earth-metal hydrides exhibit a slight decrease of stability with increase in the atomic weight of the metal, lithium hydride being the most stable member of the series. At the ordinary temperature, in absence of moisture, atmospheric oxygen, chlorine, and hydrochloric acid have no action upon it. Both types of hydride absorb hydrogen, those of the alkaline-earth-metals to a greater extent than those of the alkali-metals. All these metals combine vigorously with hydrogen, those of the alkaline earths becoming heated to incandescence, a phenomenon probably due to the greater solubility of their hydrides in the metals.

The hydride is decomposed by water without the application of external heat, lithium hydroxide being formed and hydrogen evolved. The reaction is exothermic, and is represented by the equation

[LiH]+Aq. =(H2) (dry)+LiOH,Aq.+31.6 Cal.

Since the heat of formation of lithium hydroxide in dilute solution from the metal and water is 53.2 Cal., the heat of formation of lithium hydride from its elements is given by the equation

[Li]+(H)=[LiH]+21.6 Cal., its comparatively high value according with the relative stability of the hydride. The analogy between the physical constants and other physical properties of lithium hydride and those of the alkali-metal halides, and the liberation of lithium at the cathode and hydrogen at the anode during electrolysis, indicate the hydride to be a salt of hydrogen in its capacity as a weak acid.

Lithium fluoride, LiF. - The fluoride is obtained in granular form by concentrating a hydrofluoric-acid solution of the carbonate. When crystallized from fused potassium chloride it forms regular octahedra, or leaflets with a mother-of-pearl lustre. Carnelley gives the melting-point as about 800° C., and Poulenc as about 1000° C., but Wartenberg and Schulz found 842° C. The boiling-point is 1676° C., and the vapour-pressure in atmospheres corresponds with the expression

log p = -55100/4.57T+6.190.

The density of the fluoride is about 2.6.

At 18° C. 100 parts of water dissolve 0.27 part of lithium fluoride. The salt is almost insoluble in alcohol of 95 per cent, strength, de Forcrand's value for the heat of solution is -1.04 Cal. Its comparatively slight solubility constitutes a link with the fluorides of the alkaline-earth-metals, and has been put forward as an argument in favour of the double formula Li2F2, derived from the double molecule H2F2, since the salts of lithium with monobasic anions are usually readily soluble. If this view be correct, the analogy to the alkaline- earth-metallic fluorides is rendered even more striking. It is supported by the existence of lithium hydrogen fluoride, LiF,HF, which crystallizes from a solution of the fluoride in hydrofluoric acid.

Petersen has determined the heat of formation of lithium fluoride from the hydroxide in dilute aqueous solution:

HF,Aq. +LiOH,Aq. =LiF,Aq. +16.4 Cal.

The heat of neutralization of strong acids and bases is usually about 13.7 Cal., and the enhanced value for lithium fluoride may be attributed to the heat evolved during neutralization by the ionization of the weak hydrofluoric acid.

By combining the heat of neutralization given by the foregoing equation with the heats of formation of water, dissolved lithium hydroxide, and dissolved hydrogen fluoride, an equation is obtained giving the heat of formation of dissolved lithium fluoride from lithium, fluorine, and water:

[Li]+(F)+nH2O=LiF (dissolved)+118.4 Cal.

The heat of formation of the solid fluoride is unknown, since its heat of solution has not been determined.

At red heat water has very slight action on lithium fluoride. With other fluorides the lithium salt yields double salts or complex compounds, such as BF3,LiF (Berzelius); SbF3,LiF; SbF3,LiF,HF; SiF4,2LiF,2H2O; SnF4,2LiF,2H2O.